This invention relates generally to heat shrinkable, relatively gas impermeable, thermoplastic packaging film which can be heat sealed to itself to form a flexible package. The invention relates more particularly to a pasteurizable and/or cook-in multilayer shrink film preferably comprising an outer layer, at least one barrier layer, at least one shrink layer and an innermost sealing layer which can be used for the manufacture of bags for food products in which the packaged product is thermally treated for a certain period for pasteurizing or cooking, the multilayer structure ,being shrinkable and resistant to such conditions.
There is a need in the food packaging industry for a packaging film from which bags can be made which are of improved structural soundness such that they may be fully characterized as pasteurizable and/or cook-in. Further, it is desirable to have a precooked food product which is attractively packaged inside the film within which it was precooked.
The term "pasteurizable" as used herein is intended to refer to packaging material structurally capable of withstanding exposure to pasteurizing conditions while containing a food product. It is common practice in the food industry to apply a pasteurization process to certain food products after packaging, In order to reduce the bacterial load of such products, thereby improving product freshness and extending the shelf life. Specific pasteurization requirements tend to vary by country: however, limiting conditions probably are submersion of the hermetically sealed product in water at 98.degree. C. for 1 hour. Thus, for a bag to be characterized as pasteurizable, structural integrity of the bag must be maintained during pasteurization, i.e. the bag must have superior high temperature seal strength and must be delamination resistant under such time-temperature conditions. Additionally, the packaging material should be heat shrinkable under pasteurizing conditions so as to provide an attractively packaged pasteurized food product.
The term "cook-in" as used herein is intended to refer to packaging material structurally capable of withstanding exposure to cook-in time-temperature conditions while containing a food product. Cook-in packaged foods are essentially pre-packaged, pre-cooked foods that go directly to the consumer in that configuration which may be consumed with or without re-heating. Cook-in time-temperature conditions typically refer to a long slow cooking process, for example submersion in water at up to 90.degree. C. for several hours. Such cook-in time-temperature requirements are representative of institutional cooking requirements. Submersion at 80.degree. C. for 12 hours or 90.degree. C. for 8 hours probably represent limiting cases. Under such conditions, a packaging material properly characterized as cook-in will maintain seal integrity and will be delamination resistant. Additionally, the packaging film should be heat shrinkable under these conditions so as to form a tightly fitting package and preferably should have some tendency for product adhesion to prevent "cook-out" or collection of juices between the surface of the food product and the interior surface of the packaging material.
Generalizing, there are a number of requirements for a pasteurizable, cook-in packaging material. It is the purpose of the present invention to provide a pasteurizable, cook-in packaging film meeting all of these requirements. First, bags made from such film must have seal integrity under such conditions, i.e. the heat sealed seams should resist being pulled apart during heat shrinking. As a corollary, the film should be heat sealable to itself. Second, such bags must be delamination resistant, i.e. the multilayers making up the film must not separate or blister. Third, the food contact layer of such film must qualify under the appropriate food laws and regulations for safe food contact. Fourth, the film must provide an oxygen and vapor barrier, i.e. must possess a low permeability to maintain the freshness of the food contained therein. Fifth, the film must be heat shrinkable in hot water under these time-temperature conditions, i.e. the film must possess sufficient shrink energy such that upon the packaged food product being submerged in hot water the packaging film will shrink snugly around the product contained therein, representatively about 30-50% biaxial shrinkage at about 90.degree. C. Sixth, the film should possess optical clarity, i.e. the film should not become cloudy upon exposure to these time-temperature conditions so as to maintain eye appeal of the packaged product. Seventh, if used for cooking the film should have food product adherence to restrict "cook-out" or collection of juices between the surface of the contained food product and the food contact surface of the packaging material during cook-in, thereby increasing product yield.
In general, such a multilayer film structure will have the minimal structure (sealing and food contact layer)/(shrink layer)/(barrier layer)/(abuse layer), a composite structure being required to achieve the desired composite properties of the packaging film.
A heat shrinkable, thermoplastic, barrier packaging film for making bags which has enjoyed considerable commercial success is described in U.S. Pat. No. 3,741,253 (issued on Jun. 26, 1973 to Brax et al.), which relates to a multilayer film comprising a sealing layer of an irradiated ethylene-vinyl acetate copolymer, a core layer of vinylidene chloride copolymer, and a second outside layer of an ethylene-vinyl acetate copolymer. In manufacturing this type of heat shrinkable film, a tubular orientation process is utilized wherein a primary tube of the film is biaxially oriented by stretching with internal pressure in the transverse direction and with the use of pinch rolls at different speeds in the machine direction. Then the bubble is collapsed, and the film is wound up as flattened, seamless, tubular film to be used later to make bags, e.g. either end-seal bags typically made by transversely heat sealing across the width of flattened tubing followed by severing the tubing so that the transverse seal forms the bottom of a bag, or side-seal bags in which, the transverse seals form the bag sides and one edge of the tubing forms the bag bottom.
This type of bag is typically used by placing the food product in the bag, evacuating the bag, gathering and applying a metal clip around the gathered mouth of the bag to form a hermetic seal, and then immersing the bag in a hot water bath at approximately the same temperature at which the film was stretch-oriented, typically about 71.degree. to 96.degree. C., hot water immersion being one of the quickest and most economical means of transferring sufficient heat to the film to shrink it uniformly. Alternatively, the bag may serve as a liner of a cooking mold. One problem which has been encountered is the failure of the bag seals at the bottom of the bags, when the bag is subjected to a temperature of 70.degree. C. or higher for substantial periods of time, such as experienced during a cooking or pasteurization process.
U.S. Pat. No. 4,352,702 (issued Oct. 5. 1982 to Bornstein) covers a pasteurizable shrink bag from tubular film having a layer of hydrolyzed ethylene-vinyl acetate copolymer and an interior surface layer of a polyolefin which is cross-linkable by ionizing radiation, in with the layers being directly melt-joined without an adhesive disposed therebetween and the film being irradiatively cross-linked and oriented. A second irradiation treatment is carried out on receptacles made from the film to condition their heat seals for pasteurizing conditions.
U.S. Pat. No. 4,064,296 (issued Dec. 20, 1977 to Bornstein et al.) is directed to a coextruded tubular film having a layer of a hydrolyzed ethylene-vinyl acetate copolymer layer between two other polymeric layers at least one of which being irradiatively cross-linkable, the film being irradiated and oriented.
U.S. Pat. No. 3,595,740 (issued Jul. 27, 1971 to Gerow) discloses oxygen barrier films having an interior barrier layer of a melt extrudable hydrolyzed ethylene-vinyl acetate copolymer and a heat sealing layer of an ethylene polymer or copolymer.
U.S. Pat. No. 4,469,742 (issued Sep. 4, 1984 to Oberle et al.) discloses a pasteurizable, cook-in shrink film comprising a first or sealing layer, preferably made of an ionomer characterized as a metal salt neutralized copolymer of ethylene and acrylic acid or methacrylic acid; a second or shrink layer, a third or adhesive layer, a fourth or barrier layer, a fifth or adhesive layer and a sixth or abuse layer, wherein all of said layers are radiatively cross-linked to an extent corresponding to an irradiation dosage sufficient to increase the resistance of the film layers to delamination under cook-in or pasteurizing conditions but which permits the first layer to function as a sealing layer. The sealing layer being preferably made of an ionomer characterized as a metal salt neutralized copolymer of ethylene and acrylic acid or methacrylic acid.
U.S. Pat. No. 4,888,223 (issued Dec. 19, 1989 to Sugimoto et al.) discloses a food-packaging material comprising a heat-shrinkable, gas-barrier, multilayer plastic film laminate in the form of a seamless tube, wherein the innermost, food-contacting surface has an increased wet tension strength obtained by subjecting said surface to a Corona discharge.
JP-A-02184437-A describes a multi-ply film for packaging of raw and processed meat comprising an innermost layer which is made of a polyolefin resin containing as an essential ingredient an ethylene-ethyl acrylate-maleic anhydride copolymer.
U.S. Pat. No. 4,411,919 (issued Oct. 25, 1983, to Thompson) discloses a meat adhering cook-in packaging comprising a flexible plastic container being substantially conformable to a selected meat product and having an inner meat product contacting surface of polymeric olefin having been subjected to an energetic radiation surface treatment in the presence of oxygen sufficient to cause said inner surface to adhere to the meat product during cook-in, said container having been formed from hot blown tubular film made of polyethylene, polypropylene or ethylene vinyl acetate copolymer.
GB-B-2 009 033 (issued Aug. 25, 1982, to Matsuoka et al.) describes synthetic resin films for meat packaging which is made either of an olefin resin or a vinylidene chloride resin containing substantially no functional groups, the inner surface of which is activated by means of a corona or glow discharge treatment to increase meat-adhesiveness capable of withstanding qualitative changes in the meaty surface.
U.S. Pat. No. 4,855,183 (issued Aug. 8, 1989, to Oberle) as well discloses a multiple-layer cook-in film comprising a food contact surface made of a polyamide and having been subjected to an energetic radiation surface treatment provided by a high energy electron treatment to an extent corresponding to a dosage of up to about 12 MR.
U.S. Pat. No. 4,606,922 (issued 1986 to Schirmer) relates to a method for enhancing yield of a cook-in package meat product that includes first providing an adhering cook-in container including a flexible thermoplastic envelope being substantially conformable to a contained meat product and having an inner meat-contacting surface of a selectively irradiated ionomer or a metal salt neutralized copolymer of ethylene and acrylic acid or methacrylic acid, then conforming the container about a selected meat product and cooking the package product, whereupon the inner surface of the envelope bonds to the meat product substantially to prevent cook-out of fluids. Representatively, the ionomer of the inner binding surface is Surlyn, and a typical casing or an envelope is of the structure nylon- 6/adhesive/Surlyn.
For pasteurizable and/or cook-in multilayer shrink films for use as packaging bags for food products, it is important that the innermost layer being in contact with the food product contacts the food product closely and adheres to it so that the purge or liquids exuding from the food product do not get between the meat surface and the bag wall to create an unsightly package appearance. Specifically for meat it is necessary to keep the juices and liquids within the meat so that the meat does not become dry and its weight is not diminished. On the other hand the surface of this layer being the sealing layer must provide a strong seal closing the bag into which the food product has been introduced.
The problem to be solved by the present invention therefore is the provision of a pasteurizable and/or cook-in multilayer shrink film specifically for the manufacture of bags for food products which provides close and superior adherence to the food product and at the same time an increased seal strength when heat sealing the inner surfaces of said film.
It has been found that this problem can be solved by using as the material for the innermost sealing layer of the multilayer shrink film an ethylene-butyl acrylatemaleic anhydride copolymer, which according to a preferred embodiment is subjected to a Corona treatment.